The present invention relates to a miniaturized switching power supply, and more particularly to one having a comparator with an improved input circuit incorporating a unidirectional spike filter.
A major difficulty in the miniaturization of the switching power supply is the need to use relatively large components which allow operation at relatively low frequencies. While it has been recognized that operation at higher frequencies would allow reduction of the size of certain components, the effect of the inductance of printed circuit traces introduces noise which interferes with proper operation at high frequencies. It is desirable to reduce the size of the switching power supply to approximately the size and shape of a standard IC, so that little circuit board space is required, and connection of the switching power supply is made readily by means of terminals or pins, by use of conventional assembly equipment. Reducing the size of switching power supplies to this level requires operation at much higher frequencies, but high frequency operation has not been practical with existing designs.
In conventional miniaturized switching power supplies, the inductance of printed circuit board traces and other components making up the circuit produces ringing type noise signals which degrade operation. The presence of these inductances, along with distributed capacitances throughout the circuit, cause extraneous spikes which result in noisy operation in terms of the voltage levels to be expected at any given time. This interferes with proper operation, since the voltage level must be sensed correctly in order to allow proper regulation of the output of the unit. The occurrence of a spike voltage represents an uncertainty in the true voltage level at any given time, and contributes to imprecise operation of the circuit.
As a result of these difficulties, prior switching power supplies had to confine their operation to frequency levels at which the stray inductances would not contribute more than a minimal amount of undesirable spikes which are filterable by a simple PC network. However, this limits the minimum size which can be achieved in miniaturization. In order to achieve a miniaturized switching power supply which is less than this limit, it is necessary to operate at very high frequencies in the region of one megahertz, and at these frequencies the presence of spikes and noisy operating conditions render known forms of switching power supplies generally unusable.
Accordingly, it is desirable to provide an apparatus in which spikes are eliminated, even at very high frequency operation, especially at the input of the comparator of a switching power supply, without degrading the underlying signal.
Another object of the present invention is to provide such a filter capable of operating at high frequencies, and which does not delay or impede operation of the comparator at high frequencies.